It is known from U.S. Ser. No. 3,455,710 that plaster can be given water-repellent properties by impregnation with organosiloxanes containing Si-bonded hydrogen atoms (H-siloxanes). This reference also describes the impregnation of mixtures of plaster and calcium hydroxide with H-siloxanes. Calcium hydroxide improves the water-repellent action of the H-siloxanes. However, calcium hydroxide is sparingly soluble in water and can therefore only be used in undesirably large amounts of water or as a slurry which is difficult to meter in.
The water-repellent action of H-siloxanes can also be improved by the addition of silanolates. However, large amounts of the relatively expensive silanolates are required.
In the mass production of shaped plaster articles, such as plaster wallboards and plaster/cardboard boards, a plaster slurry is usually prepared by mixing plaster with water, waterproofing agents and other additives, the slurry being subsequently dried. High drying temperatures shorten the passage times in continuous production lines. However, shaped plaster articles dried at 90.degree. C. show a step increase in water absorption in comparison with shaped plaster articles dried at a lower temperature. In such shaped plaster articles the water-proofing using H-siloxanes is hardly effective any more. Shaped plaster articles containing H-siloxanes and calcium hydroxide also show an increased water absorption if they have been dried at 90.degree. C.